This invention relates to a drive device of variable transmission ratio, particularly suitable for operating a supercharger or other accessory of a vehicle internal combustion engine.
In the vehicle field, an obstacle to the use of internal combustion engines supercharged by means of positive displacement blower driven directly by the engine instead of by the vehicle exhaust gas is the need to use a practically stepless speed-change gear for transmitting motion from the drive shaft to the blower operating shaft. This is because as the rotational speed of the drive shaft increases the rotational speed of the blower operating shaft has to be limited both in order not to subtract too much power from the vehicle internal combustion engine, and in particular in order to avoid overcharging the engine with an excessive quantity of feed air. For this purpose, numerous drive devices are known having a transmission ratio which is variable either in a stepless manner or in discrete steps within a range of predetermined values able to maintain the rotational speed of the blower operating shaft at an acceptable value for any rotational speed of the drive shaft. These devices are almost exclusively of mechanical type, and transmit motion either by complicated ratchet mechanisms or, for example, by friction wheels or variable pitch circle diameter (stepless speed-change gears of Beyer type). All variable transmission ratio drive devices of known type are however of high constructional cost, relatively poor reliability, especially when relatively high torques have to be transmitted as are required for driving a positive displacement blower, and are of relatively low mechanical efficiency.